The integrated circuit fabrication industry relies on frequent and accurate inspections of integrated circuits and tooling to ensure that the integrated circuits are fabricated properly. Because of the extremely small size of the structures of which integrated circuits are formed, the inspection techniques used must be extremely precise and accurate. Furthermore, as the size of such structures is continually decreased over time, the inspection techniques are required to keep pace by resolving smaller and smaller structures.
One method of structure inspection is an optical technique where a beam of some kind is directed toward the structure, and then the properties of the beam are analyzed as it is either reflected by or transmitted through the structure. For example, a mask or other substrate can be inspected by scanning the substrate underneath a beam that is directed at its surface. Typically, the substrate is moved in one direction within its plane, which direction of movement is defined as the X direction. The beam is then scanned back and forth across the substrate in a direction that is transverse to the direction in which the substrate is moving, which transverse direction is defined as the Y direction. In this manner, the beam makes a raster scanning pattern across the substrate, and can be directed at one time or another to impinge upon all or any desired portion of the substrate.
As mentioned above, the beam may either transmit through the substrate or be reflected by the substrate, either in whole or in part. The beam that is initially directed toward the substrate is called the primary beam herein. The transmitted beam or the reflected beam, as appropriate for the given inspection tool, is called the secondary beam herein. The secondary beam is detected by one or more sensors, which detect one or more characteristics of the secondary beam. For example, the degree to which the secondary beam is scattered as it is reflected by a substrate tends to contain information in regard to the shape and other properties of the structures formed on the substrate.
However, an inspection tool built to measure the size of structures fabricated within a certain range of dimensions may not be able to adequately measure the size of structures fabricated within a smaller range of dimensions. Thus, as structure size continually decreases with the desired increase in integrated circuit density, there is a tendency for the inspection equipment to become outdated, typically long before it is worn out or otherwise depleted. The need to replace such outdated equipment tends to generally increase the cost of fabricating integrated circuits.
One way to increase the resolution of an inspection tool is to operate the sensing mechanisms at a higher sampling frequency. However, this approach usually requires a considerable amount of work to redesign the analog electronics, system clock synchronization, and analog to digital converters. At the currently required frequencies of more than seventy-five megahertz, such system alternations are difficult and time consuming to engineer and test, perhaps requiring a year or more to accomplish.
What is needed, therefore, is a system to more rapidly extend the utile life of an inspection system.